Bit by bit

David Lindley

In the old days, when organic matter was supposed to be infused with some vital spirit that distinguished it from the cold clay of the material world, and the variety of human types and the possession of free will were in-controvertibly attributed to the powers and generosity of God, it was not so hard to understand how an embryo grew into a whole human being. It was merely another of His miracles that a tiny organic seed, infused with the requisite life-spirit, could contain all that was necessary for the creation of a new person. And if the mechanics of the process seemed complex beyond human comprehension, that only proved the necessity of divine intervention.

But even from Aristotle’s time there was a semi-scientific debate as to how God accomplished His miracle. Was the embryo a tiny person, a homunculus, that straightforwardly grew into a full-sized human being, or was it rather, as Aristotle guessed, an unformed thing from which a new person developed bit by bit, one organ appearing after another? In the last century Victorian scientists tended to picture the sperm as a homunculus, with large head and tiny body and limbs tucked up behind, and, true to the times, thought that the male contribution contained all the important stuff: the woman’s role was merely to provide a nourishing egg and a cosy place for the embryo to grow.

Modern science refuses to allow such mysterious but lazy supernatural solutions into any explanation of how the embryo grows into a human being, and because we now understand that the embryo is a single cell – more complex than a liver cell or a brain cell or a muscle cell but of the same general form – we would like to understand embryonic development in terms of the fundamental processes of cell growth and division which are behind all biological change. The egg cell, fertilised by the sperm, starts to grow and divide, grow and divide, in something like the way a single bacterium, put in a suitably nourishing medium, will proliferate. But there is an obvious difference: a bacterium generates nothing except identical bacteria, replicas of itself, whereas an egg cell manages to grow into liver and brain and muscle cells, and does so moreover in an organised way, so that the brain and liver and muscles all end up in their appropriate places, properly connected. Aristotle was broadly right: the apparently formless egg cell generates organs and limbs in some sort of sequential manner. The modern biologist would like to understand what secret internal instructions tell the embryo to do one thing after another in the correct sequence, but that is a task of daunting complexity. As soon as one begins to wonder how a single, round blob of an egg cell can generate all that it is required to generate, the development of an embryo begins to look as mysterious and miraculous as it must have done in Aristotle’s day.

As Professor Wolpert hopes to persuade us, however, the problem is beginning to yield, one step at a time, to observation and experiment. It is hardly true to say that we now know how an embryo develops to the same extent that we understand how, for example, a virus invades the body’s healthy cells. The attack of a virus is a single purposeful stroke; the growth of an embryo is an advance on many fronts, using many devices. But something coherent can be glimpsed from the pieces that biologists have assembled.